binary immiscible metal systems for preparation of borides

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Binary immiscible metal systems for preparation ofboridesTo cite this article Vladimir N Gurin et al 2009 J Phys Conf Ser 176 012012

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Binary immiscible metal systems for preparation of

borides

Vladimir N Gurin12 Ulrich Burkhardt2 and Yuri Grin2

1Ioffe Physical-Technical Institute Russian Academy of Sciences 194021 St Petersburg Russia2Max-Planck-Institut fur Chemische Physik fester Stoffe 01187 Dresden Germany

E-mail vladimirgurinmailiofferu

Abstract The binary systems with the immiscibility in the liquid state are consideredas a tool for synthesis and crystallization of borides New preparation method using twosolvents (immiscibility gap method) is compared with the conventional solution-melt methodusing one solvent (flux method) The appropriate systems are analyzed with respect to thetemperature and concentration limits of the immiscibility gap density ratio of solvents andreagents interaction of reagents formation and crystallization of compounds and allocation ofproducts in the metallic matrix

1 IntroductionRecently we reported first results on the new route of processing and investigation of interactionof elements in binary systems with immiscibility in the liquid state (immiscibility gap method)with respect to the synthesis and crystallization from two immiscible solvents L1 and L2 [1 2]In the present work the immiscibility method was applied as the tool for synthesis of metalborides Differences are discussed between the new and the conventional solution-melt methodusing one solvent L (flux method [3]) Experimental part has been already described in [2]

Analysis of the known phase diagrams [4] reveals five binary systems most suitable forexperiments under ambient conditions GaPb CdGa AlPb AlCd and PbZn The systemGaPb has an immiscibility gap but the upper immiscibility temperature (610 C) is too lowfor synthesis of refractory compounds The system CdGa is unsuitable for immiscibility gapmethod because of very low temperatures (9 C) and narrow concentration range of immiscibility(5225 at Ga) Remaining three - AlPb AlCd and PbZn - possess wide temperatureand concentration ranges of immiscibility (table 1) and are the most suitable for synthesis andcrystallization of refractory compounds The basic differences between the flux and immiscibilitygap methods are related to

(i) temperature-concentration boundary conditions for the liquid phase(ii) presence of the immiscibility range bound to a binodal(iii) density ratios of the solvents and the reagents(iv) conditions for the chemical reaction of the reagents and formation of the compounds(v) conditions of crystallization and allocation of the products in the metallic matrix

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012012 doi1010881742-65961761012012

ccopy 2009 IOP Publishing Ltd 1

Figure 1 Generalized phase diagram with an immiscibility gap

Table 1 Temperature and concentration limits of the immiscibility gap in selected systems

Binary system ∆TL1+L2=Tc-Tmono ∆CL1+L2=Cmonoip-Cipmono

(C) (at of one component)AlCd 370 = 1020 - 650 9314 = 949 - 176 (Cd)AlPb 907 = 1566 - 659 9771 = 9791 - 019 (Pb)CdGa 9 = 291 - 282 5225 = 7495 - 227 (Ga)GaPb 2974 = 610 - 3126 919 = 937 - 18 (Pb)ZnPb 3802 = 798 - 4178 937 = 94 - 03 (Pb)

2 Temperature-concentration limits of the immiscibility gapThis condition is crucial taking into account the fact that in such systems the reagents aredissolved in different solvents They react at the immiscibility border of the solvents Theinteraction is supported by the counter diffusion and convection of solvated particles of reagentswithin immiscible layers

In a generalized phase diagram (figure 1) the immiscibility area is bound to a dome-shapedcurve (a binodal) and its basis (monotectic horizontal) The temperature interval ∆T is definedby the monotectic temperature (Tmono) and the critical point of immiscibility (Tc) There aretwo types of the phase diagrams with an immiscibility range systems with monotectic andinflection points and systems with two inflection points [5] Thus the concentration range ∆Cis defined either by the monotectic point Cmono and an inflection point Cip or by two points of aninflection [5] For the elements with the melting temperature lower than Tmono an eutectic (TeCe) should be located below the monotectic reaction In such systems preparation of compoundsand their crystallization is possible in the temperature-concentration areas of ∆T = Tc - Tmono

and ∆C = Cmonoip - Cipmono The larger are ∆T and ∆C the more variable is the given systemfor synthesis in respect to the temperatures solvent ratios L1 and L2 amounts of the dissolvedreagents and conditions for crystallization of products (eg more time for growth of the largercrystals with less defects)


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Figure 2 Arrangement of the reagents (a) and reaction products (b) in the system with animmiscibility gap

3 Density ratio of the dissolved reagents and solventsIn addition to the immiscibility gap selected solvents and reagents have to fulfill the followingcondition The reagent dissolved in the upper layer should sink in the top liquid solvent but notin the bottom liquid solvent The bottom reagent should emerge in the bottom layer but not inthe top layer (figure 2a) Such density ratio allows the transport of the reagents and supportschemical reaction at the liquid phase boundary by counter diffusion and convection

For the immiscibility gap preparation of the borides the key point is the selection of anappropriate solvent for boron Eg metals with low melting temperatures such as Al (density270 g cmminus3) Zn (714) and Cd (864) are heavier than B (234) Boron dissolves only partiallyin Al and is almost insoluble in Zn or Cd Therefore the system Ca (155)Ce(678) was chosenfor synthesis of TiB2 (figure 3a) Here Ca is lighter than B and dissolves boron Titanium (450)is soluble and emerges in liquid cerium reacting with boron at phase boundary and formingrefractory boride TiB2

Syntheses in the system Ca(+B)Ce(+Ti) were carried out in a sealed tantalum crucibleThe obtained ingots were cut in two parts one was used for metallographic characterizationanother was dissolved in strongly diluted acids to isolate the crystalline products Differentshapes of crystals were observed needles (figure 4a) isometric formations (figure 4b) or staplesof plates (figure 4c) Please note that TiB2 prepared by usual flux method is usually obtainedin form of thin hexagonal plates [3] More detailed investigations on the origin of the differentcrystal shapes are in progress

Another solvent for boron can be obtained by reducing of the density of the aluminium melt byadding a lighter metal eg Mg (174) The MgAl ratio should be selected so that the density ofthe mixture is lower than the density of boron So for the target density of 22 g cmminus3 the mass


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Figure 3 Phase diagrams CaCe and AlPb with immiscibility gaps used for synthesis andcrystallization of borides

ratio for the mixture is Al Mg = 10 7 The so-obtained AlMg solvent forms an immiscibilitygap system with Pb It is possible to perform synthesis and crystallization of hexaborides of therare-earth metals in this system despite formation of intermetallic compounds between Al andMg and between Mg and Pb These compounds decompose at the temperatures between 450and 550 C and do not hinder the formation of refractory rare-earth hexaborides at elevatedtemperatures Based on these considerations single crystals of solid solutions La1minusxNdxB6 wereobtained from the immiscibility system (AlMg)(+B)Pb (+LaNd) in form of isometric crystalsneedles and plates (figure 5)

Use of the mixed solvents in immiscibility systems has special aspects Ideally the componentadded to one solvent should form also a phase diagram with an immiscibility gap with the solvent


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Figure 4 Crystals of TiB2 prepared in the system Ca(+B)Ce(+Ti) with different shapeneedles (a) isometric formations (b) and staples of plates (c)

of the second layer So addition of well-soluble Ga to Al in the system AlPb will be in favorbecause both Ga and Al form with Pb immiscibility systems having large ∆T and ∆C For thesame reasons an addition of Al to Ga in the immiscibility system GaPb will be also in favorTo the best of our knowledge this paper is the first report on the use of the mixed solvents forpreparation and crystallization with the immiscibility gap method

4 Allocation of the products in immiscibility systemsIn general several variants of synthesis and crystallization of compounds in immiscibility systemsmay be considered In the first scenario the reagents are soluble in different layers and theproduct is soluble in the top layer The dissolved reagents meet at the immiscibility border andreact forming a compound with a density lower than the density of the bottom solvent andhigher than the density of the top solvent The product moves to the top layer Under furthercooling the crystallization occurs close to the immiscibility border

In the second scenario one of the reagents is almost insoluble in the bottom solvent butemerges in it (eg Co in Pb) In this case the insoluble particles move to the immiscibilityborder by convection and react with the dissolved top reagent The possibility of the crystalgrowth depends here on the solubility of the products in one of the layers If they are insoluble


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Figure 5 Single crystals of La1minusxNdxB6 in different shapes obtained from the system(AlMg)(+B)Pb(+LaNd)

in both solvents a polycrystalline product is formed immediately at the immiscibility borderIf the product is soluble in one of the layers it crystallizes in the vicinity of the immiscibilityborder in the according layer This was observed in the system Pb(+Co)Zn(+Ge) insolublein Pb cobalt reacts with Ge from the top layer forming binary compound Co61Ge39 asymp Co3Ge2

(figure 6) In this case the reaction is complicated by the formation of the ternary byproductCo10Zn87+xGex with the solvent [2]

5 Separation of products of synthesis and crystallizationIn accordance with the first scenario described above the products in the form of crystals withdifferent shapes concentrate in the bottom part of the top layer (figure 2) Their separationin many cases is easier and faster than in case of the usual flux method The top layer of thesolidified ingot can be cut off and dissolved in an appropriate solvent which removes the matrixand does not attack the target crystals This reduces processing time and costs


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Figure 6 The microstructure of the ingot after immiscibility gap preparation in the systemPb(+Co)Zn(+Ge) Formation of intermetallic compounds is observed in the vicinity of theimmiscibility border in both layers together with the excess of elemental Ge crystallized bycooling from the Zn melt

6 ConclusionsAn appropriate choice of the binary systems with the immiscibility gap in the liquid state allowsperforming chemical reactions and crystallization of the products at the immiscibility borderKey requirements for this immiscibility gap method are solubility of the reagents in differentlayers and reasonable density ratio between the reagents and the solvents The feasibility of thenew preparation route is shown on example of synthesis and crystallization of refractory boridesTiB2 and La1minusxNdxB6

AcknowledgmentsThe authors would like to express their gratitude to Drs P Hohn M Schmidt and A Leithe-Lasper as well as to M Eckert and P Scheppan for valuable discussions and support inperforming experiments

References[1] Burkhardt U Bostrom M Schnelle W Hui Z Grin Yu and Gurin V 2003 Proc 9th Euro Conf Solid State

Chem (Stuttgart Germany) p 204[2] Burkhardt U Gurin V N and Grin Yu 2006 Develop Instit Sci Rep (MPI CPfS Dresden Germany) p 246[3] Gurin V N and Korsukova M M 1983 Prog Cryst Growth Charact 16 59[4] Massalski T B (ed) 1996 Binary Alloy Phase Diagrams 2nd ed (ASM International The Materials Information

Society) Vol 1-3[5] Anosov V Ya Ozerova M I and Fialkov Yu Ya 1976 Principles of physical chemical analysis (Moscow Nauka)

p 140 (in Russian)


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Binary immiscible metal systems for preparation of

borides

Vladimir N Gurin12 Ulrich Burkhardt2 and Yuri Grin2

1Ioffe Physical-Technical Institute Russian Academy of Sciences 194021 St Petersburg Russia2Max-Planck-Institut fur Chemische Physik fester Stoffe 01187 Dresden Germany

E-mail vladimirgurinmailiofferu

Abstract The binary systems with the immiscibility in the liquid state are consideredas a tool for synthesis and crystallization of borides New preparation method using twosolvents (immiscibility gap method) is compared with the conventional solution-melt methodusing one solvent (flux method) The appropriate systems are analyzed with respect to thetemperature and concentration limits of the immiscibility gap density ratio of solvents andreagents interaction of reagents formation and crystallization of compounds and allocation ofproducts in the metallic matrix

1 IntroductionRecently we reported first results on the new route of processing and investigation of interactionof elements in binary systems with immiscibility in the liquid state (immiscibility gap method)with respect to the synthesis and crystallization from two immiscible solvents L1 and L2 [1 2]In the present work the immiscibility method was applied as the tool for synthesis of metalborides Differences are discussed between the new and the conventional solution-melt methodusing one solvent L (flux method [3]) Experimental part has been already described in [2]

Analysis of the known phase diagrams [4] reveals five binary systems most suitable forexperiments under ambient conditions GaPb CdGa AlPb AlCd and PbZn The systemGaPb has an immiscibility gap but the upper immiscibility temperature (610 C) is too lowfor synthesis of refractory compounds The system CdGa is unsuitable for immiscibility gapmethod because of very low temperatures (9 C) and narrow concentration range of immiscibility(5225 at Ga) Remaining three - AlPb AlCd and PbZn - possess wide temperatureand concentration ranges of immiscibility (table 1) and are the most suitable for synthesis andcrystallization of refractory compounds The basic differences between the flux and immiscibilitygap methods are related to

(i) temperature-concentration boundary conditions for the liquid phase(ii) presence of the immiscibility range bound to a binodal(iii) density ratios of the solvents and the reagents(iv) conditions for the chemical reaction of the reagents and formation of the compounds(v) conditions of crystallization and allocation of the products in the metallic matrix


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binary immiscible metal systems for preparation of borides

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